For some years, it has been a known practice in the design of wood working machines of the indicated type to use inexpensive disposable knife blade inserts, so as to avoid the time-consuming task of resharpening the blades. In accordance with German Patent Application No. 25 59 406, which is relevant in a broad and general way to the approach taken in the present invention, it is already known to use profiled disposable knife blades, which are clamped into profiled mounting receptacles and held by means of a groove and conformable spring clamping arrangement, thus being more precisely positionable and more easily exchangeable than the previously-known perforated disposable knife blades.
In accordance with the aforementioned German application, a centrifugal wedge is positioned in the knife blade mounting groove with one gripping surface against the side wall of the groove and the other wedge surface against a retaining strip. Against the back side of the disposable knife blade there is a clamping strip which is flush against the other side wall of the groove. The centrifugal wedge, whose wedge angle obviously must be such that it does not spontaneously jam in the groove, is pressed in an outward direction by a spring.
For purposes of rapid changing of the knife blade, the retaining strip is movable in a diagonal direction with respect to the cylindrical cutter head, against spring pressure as shown in a working example. In order to change the knife blade and to adjust the new blade, it would seem that various means are needed. Since the positioning of the knife blade depends on the exact fitting together of several interpenetrating parts, small deviations from the correct positions can be cumulative. This type of inaccuracy is perhaps tolerable in certain kinds of wood working machines having cutting blades in accordance with Ger. Appl. No. 25 59 406. However, planing machines make much greater demands for precision.
European Patent No. A1-0 117 991 describes a cutter head for planing machines which likewise has disposable knife blades mounted in it. Each knife blade sits in a groove and is gripped by means of a self-locking centrifugal wedge which, in the sense of the rotational direction, lies directly on the knife blade. The profiling of the knife blade fits conformably with a corresponding profiling of the side wall of the groove. With this arrangement of the cutter head, after loosening the centrifugal wedge, the knife blades can be withdrawn lengthwise or inserted lengthwise by way of the end face of the cylindrical cutter head. The exact positioning takes place automatically as the result of the interlocking of the profiled surfaces. By these means, the changing of the knife blades is made very simple and can be carried out by unskilled personnel in a short time. These advantages are achieved, however, at the cost of a disadvantage. Unfortunately, the design of the groove is so complicated that it is not possible to make the cutter head body out of a single piece. Therefore, the cutter head body must be built up out of stamped sheets, which are arranged side by side on an axial tube and clamped between two end plates. This mode of construction using lamellae causes dimensional inaccuracy, further aggravated by reduced resistance to bending and by recurring imbalance problems. These problems can influence the surface quality of the workpiece, depending on the type of wood being worked on. Therefore, the range o usefulness of this type of machine is limited. A further disadvantage of the lamellar construction is the danger of crevice corrosion. Depending on the arrangement of the centrifugal wedge behind the knife blade, the critical cross-sectional area behind the centrifugal wedge is very small and this tends to cause a so-called bending up or tilting up effect.
Other methods devised to clamp knife blades in rotating cutting heads have been devised using a multiplicity of additional parts, as exemplified in German Patent Applications No. 29 16 138, 30 18 359, 32 09 445, 34 12 251, German Pat. No. 34 37 688, and European patent Appl. No. 0 065 496. In general these are disadvantageously complex in their construction, requiring various positioning screws, closely fitting rigid clamps, springs, and other disadvantageous features. As stated, the more parts the more cost in manufacture, the more care and skill needed in removing and replacing the knife blades, and the more opportunity for problems of positional inaccuracy, crevice corrosion, dirt accumulation, and freezing up or troublesome binding of the parts.